1. Field of the Disclosure
The present disclosure relates to a photosensitive composition for a display device, and more particularly, to a photosensitive composition for a black matrix of a liquid crystal display device, a black matrix having the composition, and a method of forming a black matrix using the composition.
2. Discussion of the Related Art
With focus of attention to display devices that are slim and portable, various flat panel display devices have replaced cathode ray tubes (CRTs). Among the flat panel display devices, liquid crystal display (LCD) devices have been widely used for monitors of personal computers, televisions, cellular phones or navigation systems due to their low power consumption, slimness, low manufacturing costs, and superior contacts with integrated circuits.
Since an LCD device including thin film transistors as a switching element, referred to as an active matrix LCD (AM-LCD) device and controlling on and off of each pixel, has excellent characteristics of high resolution and displaying moving images, the AM-LCD device has been widely used.
An LCD device includes an array substrate, a color filter substrate and a liquid crystal layer interposed therebetween. The LCD device is generally fabricated by performing processes of manufacturing the array substrate including thin film transistors and pixel electrodes, manufacturing the color filter substrate including color filter patterns and a common electrode, and interposing a liquid crystal layer between the substrates.
More particularly, in the array substrate, gate lines and data lines defining pixel regions are formed on a substrate. A thin film transistor is formed at each crossing portion of the gate and data lines, and a pixel electrode in each pixel region is connected to the thin film transistor. In the color filter substrate, a black matrix of a lattice shape including openings is formed on a substrate and surrounds edges of each pixel region to shield non-display areas of the array substrate. A color filter layer is formed in each opening of the black matrix and includes red, green and blue color filter patterns, which are sequentially arranged to correspond to respective pixel regions. A transparent common electrode is formed all over the black matrix and the color filter layer. The common electrode may be formed over the substrate of the array substrate.
An aperture ratio of each pixel region of the LCD device is affected by the black matrix. That is, to display images of high definition and high contrast ratio, the black matrix having a highly light-shielding property needs to be formed between the pixel regions of the red, green and blue color filter patterns. The black matrix is first formed when the color filter substrate is manufactured, and in the non-display areas, the black matrix blocks light from a backlight incident on a liquid crystal panel to thereby increase the contrast ratio.
Thus, the black matrix is required to prevent mixing of colors in three color pixel regions including the color filter patterns and to optically separate the pixel regions. Specially, in the AM-LCD device including thin film transistors, the black matrix needs to have a high light-blocking property. In addition, the black matrix separates the red, green and blue color filter patterns and blocks light going onto a channel of the thin film transistor to thereby prevent light leakage due to the light.
Generally, the black matrix is formed on a surface of the substrate by performing coating, light-exposing, developing and baking steps. Namely, a black matrix layer is formed on the substrate of the color filter substrate by applying a photosensitive composition in a predetermined thickness. Then, desired patterns are formed by performing a photolithography process that includes a light-exposing step using a photo mask for an appropriate critical dimension and a developing step. The patterns may be cured in an oven through a hard-baking step, thereby forming the black matrix on the substrate. The black matrix shields the non-display areas.
The black matrix may be formed of chromium. Since the black matrix of chromium has high reflectance, there are problems that lowering display qualities is caused and environmental pollution occurs. To solve the problems, various methods of dyeing, printing, electrodeposition, and pigment dispersion have been introduced, and the pigment dispersion method has been widely used. In the pigment dispersion method, a photopolymerizable composition including a colored pigment is coated on a transparent substrate and is exposed to light, developed and cured, thereby forming patterns of a black matrix.
By the way, to display images of high definition and excellent qualities, it is needed to reduce widths of patterns of the black matrix and improve transmittance and visibility. Especially, to develop an LCD device having 4.5 high definition (1280*720, 330 ppi) beyond those of WVGA and 2VGA, the patterns of the black matrix should have the widths less than 6 micrometers.
To do this, it is considered to decrease a pattern width of the photo mask used in a light-exposing step. However, if only the pattern width is simply decreased, the black matrix layer may not be sufficiently exposed to light because the intensity of light passing through a light-transmitting portion of the photo mask is weakened. Particularly, a lower portion of the black matrix layer is not exposed to light and is not chemically changed.
Therefore, when the black matrix layer is developed, sides of the lower portion of the black matrix layer are excessively removed, and an undercut shape is formed. The patterns of the black matrix have cross-sections of a trapezoidal shape not a rectangular shape due to the undercut shape.
Another solution is to form fine patterns of the black matrix by decreasing a distance between a light source and the black matrix layer and maintaining the intensity of light. By the way, in edge areas of the substrate, an expected amount of light is not irradiated due to interference because light is scattered and diffracted. In addition, with a large size of a substrate, the photo mask also has an increased size, and a central portion of the photo mask sags downwards as compared with side portions of the photo mask. This causes a difference in distances between the substrate and the portions of the photo mask, that is, light-exposing gaps, and the light intensities sectionally differ. Accordingly, since light is not sufficiently irradiated to and is not sufficiently hardened at the edge areas of the substrate, the black matrix layer may be excessively damaged at the edge areas by the developing solution.
Simply reducing the pattern width of the photo mask or decreasing the distance between the light source and the black matrix layer may cause some cuts, loss or taper damages of the patterns of the black matrix formed at the edge areas of the substrate. Moreover, the undercut shape is formed in the black matrix, and it is difficult to obtain uniform fine patterns all over the substrate.
Another solution is to increase developing time after the light-exposing step. However, this increases manufacturing time and lowers the productivity. In addition, patterns at the edge areas may be further damaged.